In the production of graphite electrodes, aggregate carbon particles are mixed together with a binder pitch, e.g. a coal tar pitch, which serves as a fluid matrix for the aggregate particles, providing a mix which can be easily extruded into the desired shape. The coke produced from the binder after carbonization holds the aggregate particles together to yield an electrode having desirable properties. The most important requirements for the binder are high carbon yield and an acceptable softening point behavior at conventional processing temperatures. Increasing the carbon yield generally improves the electrode properties, such as flexural strength. One way to improve the coking value of the binder is to employ so called "coking catalysts". These catalysts are added to the binder to promote the coking of the low molecular weight pitch molecules at temperatures below the distillation temperature of these molecules.
Nitro-substituted aromatic compounds are known coking catalysts for binder pitch. However, these compounds are thermally unstable and undergo highly exothermic reactions even at normal mixing temperatures, e.g. 150.degree.-180.degree. C. Consequently, they are difficult, unsafe and impractical to use. Additionally, the use of nitroaromatic compounds in the electrode mix causes the resulting stock to swell during the baking process, negating the benefit of added coking value and leading to little, if any, improvement in graphite strength.
It is therefore an important object of this invention to provide an improved binder system for use in the production of graphite electrodes.
Another object of the invention is to provide an improved binder system including a coking catalyst which is not reactive with the pitch at normal mixing temperatures.
Still another object of the invention is to provide an improved binder system containing pitch and a nitroaromatic compound which will promote coking of the binder at higher levels than heretofore possible and produce graphite electrodes having superior physical properties.
A more specific object of the invention is to provide an improved electrode mix containing aggregate carbon particles, binder pitch, a nitro-substituted aromatic compound as a coking catalyst and an inhibitor for reducing the tendency of the "green" electrode to swell during the baking process.
Yet another object of the invention is to provide an improved method for producing graphite electrodes by shaping a mixture of aggregate carbon particles, binder pitch and a coking catalyst which is more thermally stable at mixing temperatures, and an inhibitor to prevent swelling of the "green" electrode during baking.